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141.
Haçan Awad François Trécourt Guy Quéguiner Francis Marsais 《Tetrahedron letters》2004,45(42):7873-7877
4-Chloropyridine was deprotonated on treatment with 1/3 equiv of the highly coordinated magnesate Bu3(TMP)MgLi2 in THF at −10 °C, as evidenced by trapping with I2. The use of Bu(TMP)2MgLi in Et2O allowed the reaction of 2-chloropyridine, giving the 3-functionalized derivative as the main product. Mixtures of 3- and 4-functionalized derivatives were obtained when 2,6-dichloropyridine was involved in the reaction. Performing the reaction on 3-chloropyridine with lithium magnesates in THF, either the 4,4′-dimer or the 4-iodo derivative was formed after quenching by I2, the former using 1/3 equiv of Bu2(TMP)MgLi and the latter using 1 equiv of (TMP)3MgLi. Similar results were observed with 3,5-dichloropyridine, 2,5-dichloropyridine and 3-chloro-2-fluoropyridine. 1,2-Migration of the lithium arylmagnesate formed by deprotonation was proposed to justify the dimers formation. 相似文献
142.
Using a modification of the Kharasch-Sosnovsky reaction, the oxidation of oxazolines and thiazolines bearing a variety of 2-alkyl substituents (chiral and achiral) were smoothly oxidized to their corresponding oxazoles and thiazoles, respectively. The key feature involved in the successful implementation of this important oxidation was the use of a mixture of Cu(I) and Cu(II) salts to enhance the oxidation of the intermediate captodative radical, 24. The main limitation of this method was shown when the oxidation failed with oxazolines/thiazolines lacking the carboalkoxy group at C-4. 相似文献
143.
The composition of cellulose, hemicellulose, starch, lignin, pectin, protein, and total lipid content in the selected cellulosic
wastes-tapioca (Manihot esculenta) stem, leaf, petiole, and water hyacinth (Eichhornia crassipes) were determined. The effectiveness of various physical and chemical pretreatments on the enzymatic digestibility of these
wastes were identified. In general, chemical pretreatments were more effective than physical pretreatments. The efficiency
of the pretreatment was checked by subjecting these wastes to enzymatic saccharification after the pretreatments. 相似文献
144.
145.
Tam F Syrstad EA Chen X Turecek F 《European journal of mass spectrometry (Chichester, England)》2004,10(6):869-880
Protonated acetamide exists as two planar conformers, the more stable anti-form (anti-1(+)) and the syn-form (syn-1(+)), DeltaG(degree) (298) (anti-->syn) = 10.8 kJ mol(-1). Collisional neutralization of 1(+) produces 1-hydroxy-1-amino-1-ethyl radicals (anti-1 and syn-1) which in part survive for 3.7 micros. The major dissociation of 1 is loss of the hydroxyl hydrogen atom (approximately 95%) which is accompanied by loss of one of the methyl hydrogen atoms (approximately 3%) and loss of the methyl group (approximately 2%). The most favorable dissociation of the OH bond is calculated to be only 34 kJ mol(1) endothermic but requires 88 kJ mol(-1) in the transition state. Other dissociations of 1, e.g., loss of one of the amide hydrogens, methyl hydrogens, and loss of ammonia are calculated to proceed through higher- energy transition states and are not kinetically competitive if proceeding from the ground doublet electronic state of 1. The unimolecular dissociation of 1 following collisional electron transfer is promoted by large Franck-Condon effects that result in 8090 kJ mol(-1) vibrational excitation in the radicals. Radicals 1 are calculated to exoergically abstract hydrogen atoms from acetamide in water, but not in the gas phase. The different reactivity is due to solvent effects that favor the products, (.)CH(2)CONH(2) and CH(3)CH(OH)NH(2), over the reactants. 相似文献
146.
Nie HY Miller DJ Francis JT Walzak MJ McIntyre NS 《Langmuir : the ACS journal of surfaces and colloids》2005,21(7):2773-2778
As determined by scratch tests, self-assembled monolayers (SAMs) of octadecylphosphonic acid (OPA) on a muscovite mica substrate were found to be mechanically robust and to serve as a lubricant to protect the underlying mica substrate. For comparison purposes, three polymer films were subjected to scratch tests under the same conditions. The scratch tests were conducted using a diamond-tipped stylus, and the resultant scratches were examined using atomic force microscopy. The excellent mechanical strength of OPA SAMs is supported by analysis with time-of-flight secondary ion mass spectrometry, which suggests that the headgroup of the OPA is strongly bonded to the substrate atoms. The molecular lubrication provided by OPA SAMs suggests that the interaction between the headgroup and the substrate is sufficiently strong to endure significant shear force and that the hydrocarbon chains are able to dissipate shear energy. 相似文献
147.
The quenching of fluorescence of the free-base tetraphenylporphyrin, H2TPP, and its metal derivatives, MgTPP and ZnTPP by diverse iron(III) complexes, [Fe(CN)6]3−, Fe(acac)3, [Fe(mnt)2]−, Fe(Salen)Cl, [Fe4S4(SPh)4]2−·, FeTPPCl and [Fe(Cp)2]+ has been studied both in homogeneous medium (CH3CN) and micellar media, SDS., CTAB and Triton X-100. The quenching efficiencies are analysed in terms of diffusional encounters
and it has been possible to separate static quenching components. The quenching constants are dependent on the nature of the
ligating atoms around iron(III) and also on the extent of π-conjugation of the ligands. The quenching mechanism has been investigated
using steady-state irradiation experiments. Evidence for oxidative quenching by iron(III) complexes was obtained, though the
spin multiplicities of the excited electronic states of iron(III) complexes permit both energy and electron transfer mechanisms
for quenching of the singlet excited state of the porphyrins. 相似文献
148.
149.
Moon FC 《Chaos (Woodbury, N.Y.)》1991,1(1):31-41
In this paper a review is given of experimental techniques in chaotic dynamics of solid mechanical systems based on modern ideas of nonlinear dynamics. These methods include Poincare maps, double Poincare sections, symbol dynamics, and fractal dimension. The physical problems discussed include nonlinear elastic beams, forced motion of a string, flow-induced vibration of a rod, forced motions of a magnetic pendulum, and rigid body dynamics of a magnet and high-temperature superconductor. 相似文献
150.
Summary The CHARGE2 program for the calculation of partial atomic charges has been amended to include bond parameters for a number of organic functional groups, including halogens, nitrogen and oxygen. These minor amendments to the original scheme produce dipole moments for the fluoro and chloro compounds which are in complete agreement with the observed values.The less complete data sets for the bromo and iodo compounds are also well reproduced, and the dipole moments of a variety of mixed halo compounds are now in better agreement with experiment than previously.The calculated dipole moments of the saturated nitrogen and oxygen compounds are now in much better agreement than in the original scheme, thus the revised parameterisation may be employed with confidence to predict the electrostatic energies of these compounds.Furthermore, the revised scheme now gives a precise proportionality between the charge on the proton in a CH group and the 1H chemical shift of the corresponding proton, allowing the general prediction, in principle, of 1H chemical shifts. In addition, attempts to include variable electronegativity in the effect are described for fluoro compounds.For part VIII see Ref. 1. 相似文献